1. FIELD OF THE INVENTION
The invention relates to a hydraulic lifting mechanism, particularly for an industrial truck, which includes a hydraulic lifting cylinder and a hydraulic unit used as a pump in a load lifting operation. The hydraulic unit supplies a pressure medium to the lifting cylinder and works as a motor in a load lowering operation actuated by the pressure medium supplied by the lifting cylinder. A DC generator is coupled with the hydraulic unit and functions as an electric motor in the load lifting operation and as a generator in the load lowering operation, and includes a regenerative brake circuit powered by the DC generator in the load lowering operation. A control valve arrangement is located in the center of the pressure path between the hydraulic cylinder and the hydraulic unit, and a lifting mechanism control regulates the regenerative brake circuit and the control valve arrangement.
2. DESCRIPTION OF THE RELATED ART
A hydraulic lifting mechanism of this type is known from the DE-OS No. 20 14 605. In this lifing mechanism for a forklift, the pump, supplying the hydraulic lifting cylinder with hydraulic fluid, is driven by a DC shunt motor powered by the battery of the vehicle, the lifting cylinder lifting the load. The pump is a rotary piston pump whose output quantity can be reset steplessly from pump operation to motor operation by actuation of a regulator. When lowering the load, the pump is operated as a hydraulic motor, which drives the DC shunt motor operating as a generator in a regenerative braking operation. The potential charging energy of the lifted load is converted into electric energy charging the battery during lowering of the load in the regenerative braking operation.
In the lifting mechanism of the previously explained type, the lowering speed of the load is determined by the braking torque produced by the DC shunt motor. Since the DC shunt motor does not produce a braking torque when it is not operating, a controllable shut-off valve is installed between the hydraulic motor and the hydraulic cylinder, which, in the blocked state, shuts off the hydraulic fluid flow from the hydraulic cylinder to the hydraulic motor. When the load is lowered from the stopped position, this shut-off valve is opened. In conventional lifing mechanisms of the type under discussion, the load hereupon drops while accelerating the generator, until the generator produces an adequate counter torque in order to stop the load.